Toxins have historically been considered to be stable preparations; however, with the preparation of combination vaccines and the production of antibodies or antitoxins by these vaccines, the characterization and subsequent stability of toxins and antitoxins has become more relevant. Examination of diphtheria toxin stability in the Vero cell test was implemented this year using a modified assay that included freeze-dried antitoxins at specific set potencies. Using this method, toxin were found to be very stable at 2-8 degrees C and with freezing, but lost potency after thawing and refreezing. Similar studies are underway with the Antitoxin for Diphtheria; again in the cell assay, it appears that freezing is more detrimental to the product than storage at 2-8 degrees C. This finding has been utilized to insure the stability of the toxin preparation used in the cell assay. Electrophoretic studies had previously focused on elevated temperature; we plan to evaluate the toxin and antitoxin known to have failed in the newly revamped Vero cell neutralization assay.